This invention relates to the hydrocarboxylation of olefins to form carboxylic acids. More specifically, this invention relates to the hydrocarboxylation of propylene with CO and H.sub.2 O in the liquid phase to produce butyric acid, wherein the isobutyric acid isomer product predominates. The isobutyric acid may then be dehydrogenated to produce methacrylic acid.
Conventionally, hydrocarboxylation of olefins has been intended to produce predominantly the linear, straight-chain or normal (n) isomer of the carboxylic acid desired. U.S. Pat. No. 3,641,074 to Fenton discloses the preparation of normal or straight chained carboxylic acids, esters and anhydrides via the carbonylation of olefins in the presence of a Group VIII noble metal in complex association with a biphyllic ligand. Suitable ligands may include triarylphosphine and triarylarsine among others, and suitable solvents may include hydrocarbons, acids, ketones, ethers and esters, among others.
U.S. Pat. No. 3,816,490 to Forster et al. discloses the production of carboxylic acids by carboxylation of olefins, utilizing a Group VIII metal compound, preferably cobalt, rhodium and iridium together with a phenolic promoter compound. The metal compound may be elemental metal, a simple salt, or an organometallic complex. Acetic acid and propionic acid are disclosed as solvents for the reaction, which yields predominantly normal carboxylic acids when isomeric products are to be prepared.
U.S. Pat. Nos. 3,857,900 and 3,933,919 to Wilkinson disclose hydrogenation, hydroformylation and carbonylation reactions resulting primarily in the formation of linear products when catalysts comprising platinum group metals, ligands containing nitrogen, phosphorus, arsenic or antimony; and a halogen or pseudo-halogen are utilized.
U.S. Pat. Nos. 3,919,272 and 3,968,133 to Knifton disclose the preparation of linear fatty acids and esters from olefins, carbon monoxide and alcohols or water in the presence of ligand-stabilized palladium halide complexes in combination with a halide salt of either tin or germanium. Ligands may include phosphines, and arsines among others. Suitable solvents include aromatic hydrocarbons, ketones, ethers and chlorinated olefins ('272).
The preparation of increased ratios of branched-chain or iso-carboxylic acids to straight-chain acids is described in U.S. Pat. No. 3,661,949 to Fenton. Olefins are hydrocarboxylated in the presence of a biphyllic ligand-stabilized Group VIII noble metal compound catalyst and an iron halide co-catalyst. The ligand may include arsines or phosphines, and suitable solvents include hydrocarbons, ketones, ethers, and acids.
West German Offenlegungsschrift No. 2,739,096 describes the preparation of isobutyric acid esters by the carbonylation of propylene with carbon monoxide and an alcohol in the presence of a palladium salt, a triarylarsine, a halogen acid. Solvents for the reaction include aromatic hydrocarbons, substituted aromatics, chlorinated hydrocarbons, ethers and sulfones. An amount of water larger than about 0.1 mole per mole of propylene is described as being harmful to the carbonylation reaction.
In general the single metal salt catalyst systems are non-selective for the "iso" form of the carboxylic acid products, tending to yield predominantly straight chain products. The catalyst systems described above for use in hydrocarboxylation are often susceptible to deactivation due to the reduction of the Group VIII metal ion particularly palladium, contained therein to the elemental state by the action of the carbon monoxide reactant. These systems may also present difficulties in the separation of the solubilized catalyst from the liquid reaction products. Any catalyst system to be utilized in the hydrocarboxylation reaction must be thermally stable at the temperatures required for the reaction to effectively occur. Other factors which effect the hydrocarboxylation reaction are the solvent used, and the molar ratios of catalyst to stabilizing ligands, to the reactants, and to other components of the system.